Heretofore, there has been known a reclining mechanism for a seat such as a vehicle seat, as disclosed, for example, in Japanese Utility Model Laid-Open Publication No. 05-001335 (hereinafter referred to as “Patent Document 1”). A reclining mechanism disclosed in the Patent Document 1 comprises: a base plate (first plate) and an arm plate (second plate) coaxially supported in a rotatable manner and formed, respectively, with two opposed circular holes having an internal gear (internally-toothed gear) section thereinside; a control gear provided on a rotary shaft of the two plates; a plurality of planetary gears arranged in meshing engagement with the internal gear section of the plates and the control gear, and each protrudingly provided with a boss portion at an axis thereof; and a holding member rotatably holding each of the planetary gears at a trajectory of revolution thereof. This holding member is composed of a spring member which is adapted to contact the respective boss portions of the planetary gears while permitting a rotation thereof, and bias each of the planetary gears in a radial direction (in a direction causing the planetary gear to be brought in meshing engagement with the internal gear section of the plates. This reclining mechanism can suppress wobbling due to a backlash between the internal gear section of the two plates (base and arm plates) and each of the planetary gears, etc.
However, even in the case where the planetary gears are biased by the holding member composed of a spring member as described in the Patent Document 1, for example, when a force equal to or greater than a biasing force of the holding member is applied to a seat back, the planetary gears are likely to be displaced radially inwardly against the biasing force of the holding member, causing wobbling between the internal gear section of the plates and each of the planetary gears. On the other hand, if the biasing force of the holding member is increased, a rotational resistance of each of the planetary gears becomes larger, precluding smooth rotation thereof. As above, the technique using the holding member composed of a spring member as in the Patent Document 1 has a problem of being unable to sufficiently suppress wobbling between the internal gear section of the plates and each of the planetary gears.
Moreover, in the reclining mechanism disclosed in the Patent Document 1, when a force is repeatedly applied to the seat back in a rearward tilting direction thereof, each of the planetary gears is likely to being slightly rollingly displaced with respect to the internal gear section of the plates. If this situation occurs, the seat back will be gradually tilted rearwardly with respect to a seat cushion, thereby making it impossible to maintain the seat back in a fixed state.